DOI: 10.3303/CET25117110 Paper Received: 31 December 2024; Revised: 18 March 2025; Accepted: 29 May 2025 Please cite this article as: Kobayashi D., Watanabe H., 2025, Improvement of Phenol Degradation by UV Irradiation on Ultrasonic Atomized Droplets , Chemical Engineering Transactions, 117, 655-660 DOI:10.3303/CET25117110 CHEMICAL ENGINEERING TRANSACTIONS VOL. 117, 2025 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Fabrizio Bezzo, Flavio Manenti, Gabriele Pannocchia, Almerinda di Benedetto Copyright © 2025, AIDIC Servizi S.r.l. ISBN 979-12-81206-17-5; ISSN 2283-9216 Improvement of Phenol Degradation by UV Irradiation on Ultrasonic Atomized Droplets Daisuke Kobayashi*, Hinano Watanabe Department of Applied Chemistry, Tokyo Denki University, Adachi, Tokyo 120-8551, Japan kobayashi@mail.dendai.ac.jp Photocatalysis such as TiO2 particles are expected to be applied to the degradation of hazardous organic materials as an advanced oxidation processes (AOPs). However, UV light is shielded by the catalyst particles, so the area of catalytic activity is limited spatially. On the other hand, ultrasonic atomization has been reported to produce very fine droplets from bulk solutions, and to disperse particles in liquids into droplets. Therefore, it is expected that the combination of ultrasonic atomization and photocatalysis enables a higher concentration of photocatalytic particles in the atomized droplets. In this study, droplets containing TiO2 were generated using an ultrasonic atomizer, and phenol degradation was carried out by UV light irradiation to the droplets. Kinetic analysis was performed by assuming the reaction to be a pseudo-first-order reaction, and the effects of particle concentration and particle size on the degradation rate constants were investigated. The degradation rate constant increased with increasing TiO2 particle addition. Phenol degradation by photocatalytic particles and UV irradiation was also investigated. The results were compared with those obtained with a device that uses a combination of ultrasonic atomization and UV irradiation. At the same TiO2 particle concentration, the degradation rate of the device that combined ultrasonic atomization and UV irradiation was higher than that of the device that used UV irradiation. It is assumed that the equipment using a combination of ultrasonic atomization and UV irradiation increased the degradation rate because TiO2 particles were concentrated and incorporated into the droplets, resulting in a higher particle concentration. It is estimated that TiO2 particle concentration inside the droplet is twice as concentrated as evaluated from the results of degradation rate constant. 1. Introduction Photocatalysis such as TiO2 particles are expected to be applied to the degradation of hazardous organic materials as an advanced oxidation processes (AOPs). Catalytic activity, such as radical formation, is exhibited by energy sources such as UV light irradiation (Kormann et al., 1991). However, light is shielded by the added particles. A method that combines ultrasound and photocatalytic particles, which are not affected by the shielding effect of the particles, is investigated (Kubo et al., 2005, Park et al., 2011). Sekiguchi and Saita (2001) have also investigated the effect of alumina particle addition on ultrasonic degradation of organic materials. Degradation using ultrasound is strongly depend on the frequency. Kobayashi et al. (2012, 2014) have investigated the effects of ultrasonic frequency on degradation of methylene blue, and proposed a model for estimating rate constants using sonochemical efficiency. Honma et al. (2013) and Kobayashi et al. (2019) have reported that the enhancement of degradation rate by particle addition was influenced by both ultrasonic frequency and type or diameter of particles. Combination of ultrasonic atomization and UV is also investigated for degradation of hazardous organic materials. Itoh et al. (2019) have reported that KI oxidation by simultaneous treatment of US/UV was higher than the sum of respective rate constants obtained by US treatment alone and UV treatment alone. It was suggested that oxidation reactions are promoted by catalytic reactions not only in solution but also in the fine mist generated by ultrasonic atomization. Kato et al. (2021) also proposed a wastewater treatment system using mist and photocatalyst, and the presence of mist in the reactor has been revealed to increase the removal rate of pollutants (Kato et al., 2022). Kato et al. (2023) have reported that US irradiation generates mist and UV 655 irradiation and TiO2 promote photocatalytic removal of phenol in the mist. However, the reasons for the synergistic effect of organic degradation by the combined use of atomized mist and UV have not been sufficiently clarified. On the other hand, ultrasonic atomization has been reported to produce very fine droplets from bulk solutions, and to disperse particles in liquids into droplets (Suzuki et al., 2012, Nii et al., 2014). It has been reported that irradiating a sample suspension with 2.4 MHz ultrasound is able to separate particles of 90-320 nm, regardless of the type of material. Therefore, ultrasonic atomization has potential for use in particle classification as shown in Figure 1. Using ultrasonic atomization to capture particles in mist, the combined use of UV irradiation and ultrasonic atomization is to be expected to improve the efficiency of the organic material degradation process. In this study, droplets containing TiO2 were generated using an ultrasonic atomizer, and phenol degradation was carried out by UV light irradiation to the droplets. A higher concentration of photocatalytic particles in the atomized droplets is expected. For comparison, phenol degradation with TiO2 and UV light is also performed to estimate particle concentration in the mist. The purpose of this study is to investigate the effect of particle concentration and particle size on the degradation rate of phenol. Figure 1: Concept of particle classification by ultrasonic atomization 2. Experimental 2.1 Materials Phenol (99.0 %, Kanto Chemical Co., Inc.) was used as a model material for hazardous organic compounds, and TiO2 of Anatase form (98.5 %, Kanto Chemical Co., Inc.) was selected as the photocatalysis. Acetonitrile (99.9 %, Kanto Chemical Co., Inc.) was used for HPLC analysis. 2.2 Apparatus The experimental apparatus with a combination of ultrasonic atomization and UV is shown in Figure 2. Phenol aqueous solution of 50×10-3 dm3 with the concentration of 1 mmol dm-3 was added to a reactor. Outer dimensions of the acrylic reactor were 70 mm in length, 70 mm in width, and 200 mm in height. An ultrasonic transducer with a frequency of 1.6 MHz was attached to the bottom of the reactor. TiO2 particles were suspended in the solution. The UV lamp (16 W, LUV-16, AS ONE Corporation) has a main wavelength of 365 nm. The atomized mist was irradiated with UV light from the side of the reactor. The distance between the reactor wall and the UV light was approximately 100 mm. Light irradiation was performed at this position because UV irradiation makes the wall of the reactor hot. The experimental setup for UV alone is shown in Figure 3. Phenol aqueous solution of 50×10-3 dm3 with the concentration of 1 mmol dm-3 was added to a glass reactor. A stirrer was placed in the reactor and stirred at 350 rpm with a magnetic stirrer. UV light was irradiated from the top of the reactor. The distance between the sample liquid surface and the UV light was approximately 50 mm. 656 Figure 2: Experimental apparatus with a combination of ultrasonic atomization and UV Figure 3: Experimental apparatus for UV alone 2.3 Experimental conditions Initial concentration of phenol (C0) was fixed at 1 mmol dm-3. Reaction time (t), concentration of TiO2 particles (CP), and diameter of TiO2 particles were changed. Table 1 shows the operating conditions. Table 1: Operational conditions frequency f [kHz] Initial concentration of phenol C0 [mmol dm-3] Reaction time t [min] Concentration of TiO2 CP [g dm-3] Diameter of TiO2 [nm] 1.6 1.0 0 – 90 0 – 0.5 10 – 30 100 – 300 2.4 Analysis Concentration of phenol (C) was determined using high performance liquid chromatography (HPLC) system. After the reaction, particles were removed by centrifugation and analyzed by HPLC. Luna Omega Polar C18 was used as the column. The column temperature was set at 30°C, the mobile phase was acetonitrile and pure water (3:7), and the flow rate was 0.8 mL/min. In previous studies, phenol degradation by ultrasound or photocatalysis is often considered a pseudo-first-order reaction. In this study, we thought a simple model would be preferable to investigate the effect of operating conditions on the degradation rate. Therefore, phenol degradation was assumed to proceed in a pseudo-first-order reaction, and degradation rate constants (kapp) were estimated using Eq(1) and the temporal change of phenol concentration. 𝐶 = 𝐶0 exp(−𝑘app𝑡) (1) UV 657 3. Results and Discussion 3.1 Effects of TiO2 concentration on degradation of phenol The degradation rate was measured by ultrasonic irradiation without the addition of titanium dioxide particles in order to investigate the effect of degradation by ultrasound alone. As a result, the degradation rate constant was calculated to be 0.0003 min-1. It has been revealed that oxidative degradation by hydroxyl radicals is the dominant mechanism of degradation of phenol by ultrasound (Shimakage et al., 2016). It is expected that the contribution of phenol degradation by ultrasound is negligible because the sonochemical efficiency (Koda et al., 2003) is low at the frequency used in the present study. Figure 4 shows the effects of TiO2 concentration on the degradation rate constant in the experimental using a combination of ultrasonic atomization and UV light irradiation. When particle size is small, the degradation rate constant increases even at higher particle concentrations. However, as the particle concentration increases above 0.1 g/L, the rate constant approaches a constant value. On the other hand, when the particle size was large, the maximum degradation rate constant occurred at a particle concentration of 0.05 g dm-3. In addition, the degradation rate decreased with increasing particle concentration. When the particle size was large, the accumulation of particles on the transducer at the bottom of the reactor was observed. It is suggested that the degradation rate constant decreased due to the accumulation of particles on the transducer, which reduced the amount of atomized mist. Figure 4: Effects of TiO2 concentration on the degradation rate constant in the experimental using a combination of ultrasonic atomization and UV light irradiation (a) 100 – 300 nm, (b) 10 – 30 nm 3.2 Effects of TiO2 concentration on degradation of phenol Figure 5 shows the effects of particle size on degradation rate constant in the experimental using a combination of ultrasonic atomization and UV light irradiation. TiO2 particle concentration was 0.05 g dm-3. The degradation rate constant was about four times higher for the large particle size compared to the small particle size. In the present system, it is considered that particles between 100 and 300 nm were more easily incorporated into the atomized mist. Since the ultrasonic operating conditions, such as ultrasonic frequency, were not changed in this study, there is no way to know what factors affect particle incorporation into the atomized droplets. However, it 0 0.002 0.004 0.006 0 0.05 0.1 0.15 D eg ra d at io n r at e co n st an t [m in -1 ] Concentration of TiO2 [g dm-3] 0 0.002 0.004 0.006 0 0.2 0.4 0.6 Concentration of TiO2 [g dm-3] D eg ra d at io n r at e co n st an t [m in -1 ] 658 is necessary to investigate the relationship between the size of particles that are easily incorporated into droplets formed by ultrasonic atomization and the operating conditions. Figure 5: Effects of particle size on the degradation rate constant in the experimental using a combination of ultrasonic atomization and UV light irradiation 3.3 Effects of TiO2 concentration on degradation of phenol Figure 6 shows the effects of TiO2 concentration on the degradation rate constant in the experimental using UV light alone system. TiO2 particle size was 100 – 300 nm. At a particle concentration of 0.05 g dm-3, where the maximum degradation rate constant is obtained with the system using both ultrasound and UV light, the rate constant is 0.002 min-1 with UV alone. However, the degradation rate constant in the combined system is about 0.005 min-1. It is revealed that the combined use of UV light and ultrasonic atomization has a synergistic effect and improves degradation, because ultrasound alone does not degrade most of the materials. The particle concentration in the mist formed by ultrasonic atomization is estimated around 0.12 g dm-3 from the results of degradation rate constants. It was also suggested that TiO2 were incorporated into the atomized droplets, and the concentration of particles in the droplets increased. The combined use of ultrasonic atomization and UV light irradiation make the reaction more efficient, suggesting the possibility of process intensification. Ultrasonic atomization can also be applied to continuous operation by circulating bulk gas to capture the mist. Therefore, it can be applied to large-scale processing by continuous operation and is expected to achieve industrial application. Figure 6: Effects of TiO2 concentration on the degradation rate constant in the experimental using UV light alone system 4. Conclusions Degradation of phenol was performed in an apparatus that used a combination of ultrasonic atomization and UV light irradiation, and the effects of operating conditions on the degradation rate constant, were examined. 0 0.002 0.004 0.006 D eg ra d at io n r at e co n st an t [m in -1 ] Particle size large small 0 0.002 0.004 0.006 0.008 0 0.05 0.1 0.15 0.2 D eg ra d at io n r at e co n st an t [m in -1 ] Concentration of TiO2 [g dm-3] 659 It is assumed that the equipment using a combination of ultrasonic atomization and UV irradiation increased the degradation rate because TiO2 particle was concentrated and incorporated into the atomized mist droplets, resulting in a higher particle concentration. Nomenclature C – concentration of phenol, mmol dm-3 C0 – initial concentration of phenol, mmol dm-3 CP – concentration of TiO2, g dm-3 f – ultrasonic frequency, kHz kapp – degradation rate constant, min-1 t – reaction time, min Acknowledgments This work was supported by JSPS KAKENHI Grant Number JP20K05212. References Honma, C., Kobayashi, D., Matsumoto, H., Takahashi, T., Kuroda, C., Otake K., Shono, A., 2013, Effect of particle addition on degradation rate of methylene blue in an ultrasonic field, Japanese Journal of Applied Physics, 52, 07HE11. Itoh, T., Kojima, Y., 2019, Synergistic effects of ultrasound and ultraviolet light irradiation on oxidation reaction using photocatalyst, Journal of Chemical Engineering of Japan, 52, 877-881. Kato, S., Sakai, Y., Sato, Y., Kansha, Y., 2021, The effect of high-frequency ultrasound on the photocatalytic decomposition of organic compounds in water, Chemical Engineering Transactions, 88, 379-384. Kato, S., Sakai, Y., Sato, Y., Kansha, Y., 2022, The effect of the presence of mist in the proposed sonophotocatalytic wastewater treatment process, Chemical Engineering Transactions, 94, 583-588. Kato, S., Sakai, Y., Sato, Y., Kansha, Y., 2023, Enhancement of wastewater treatment using mist and photocatalyst, Chemical Engineering Technology, 46, 1185-1190. Kobayashi, D., Honma, C., Matsumoto, H., Takahashi, T., Kuroda, C., Otake, K., Shono, A., 2014, Kinetics analysis for development of a rate constant estimation model for ultrasonic degradation reaction of methylene blue, Ultrasonics Sonochemistry, 21, 1489-1495. Kobayashi, D., Honma, C., Suzuki, A., Takahashi, T., Matsumoto, H., Kuroda, C., Otake, K., Shono, A., 2012, Comparison of ultrasonic degradation rates constants of methylene blue at 22.8 kHz, 127 kHz, and 490 kHz, Ultrasonics Sonochemistry, 19, 745-749. Kobayashi, D., Matsumoto, H., 2019, Kinetics analysis for development of a rate constant estimation model for ultrasonic degradation reaction in the presence of particles, Chemical Engineering Transactions, 74, 571- 576. Koda, S., Kimura, T., Kondo, T., Mitome, H., 2003, A standard method to calibrate sonochemical efficiency of an individual reaction system, Ultrasonics Sonochemistry, 10, 149-156. Kormann, C., Bahnemann, D. W., Hoffmann, M. R., 1991, Photolysis of chloroform and other organic molecules in aqueous titanium dioxide suspensions, Environmental Science & Technology, 25, 494-500. Kubo, M., Matsuoka, K., Takahashi, A., Shibasaki-Kitakawa, N., Yonemoto, T., Kinetics of ultrasonic degradation of phenol in the presence of TiO2 particles, Ultrasonics Sonochemistry, 12, 263-269. Lee, B.-N., Liaw, W.-D., Lou, J.-C., 1999, Photocatalytic decolorization of methylene blue in aqueous TiO2 Suspension, Environmental Engineering Science, 16, 165-175. Nii, S., Oka, N., 2014, ize-selective separation of submicron particles in suspensions with ultrasonic atomization, Ultrasonics Sonochemistry, 21, 2032-2036. Park, B., Cho, E., Park, H., Khim, J., 2011, Sonophotocatalytic destruction of chloroform: Comparison of processes and synergistic effects, Japanese Journal of Applied Physics, 50, 07HE10. Sekiguchi, H, Saita, Y., 2001, Effect of alumina particles on sonolysis degradation of chlorobenzene in aqueous solution, Journal of Chemical Engineering of Japan, 34, 1045-1048. Shimakage, K., Kobayashi, D., Naya, M., Matsumoto, H., Shimada, Y., Otake, K., Shono, A., 2016, Effect of dimethyl sulfoxide addition on ultrasonic degradation of methylene blue, Japanese Journal of Applied Physics, 55, 07KE01. Suzuki, K., Hisaeda, J., Nii, S., 2012, Application of ultrasonic atomization for fractionating particles in suspensions, Journal of Chemical Engineering of Japan, 45, 114-118. 660 CET-vol117-b.pdf 315kobayashi.pdf Improvement of Phenol Degradation by UV Irradiation on Ultrasonic Atomized Droplets